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Terminal Unit
Control (TUC)
94 UNT-IOM-6
The entering water temperature sampling function allows water to
circulate in an attempt to sense water cold enough to provide cooling.
The function uses a timer to permit the controller and the valve to fully
open while the water circulates for 3 minutes. The fan continues to run
at the selected speed. If the changeover sensor does not sense the
correct water temperature within this 3 minute cycle, the valve closes
and the entering water temperature sampling function will cease until
57 minutes elapse. After this time lapse, the water circulates again
through the unit to determine if the entering water temperature indi-
cates a call for cooling or heating.
Note: The entering water temperature sampling function is for units
with 2-way valves. This function is not necessary for units with 3-way
valves since water flows continuously through this piping configura-
tion.
2-Pipe with
Auxiliary Electric
Heat (Fan-coils)
Entering Water
Temperature
Sampling Function
Table 38. Unit Mode as Related to Water Temperature
Unit Type EWT Sensor Required? Coil Water Temperature
2-pipe changeover Yes · Can cool if:
space temp - EWT
³ 5 deg F
· Can heat if:
EWT - space temp
³ 5 deg F
4-pipe changeover Yes · Can cool if:
space temp - EWT ³ 5 deg F
· Can heat if:
EWT - space temp
³ 5 deg F
2-pipe heating only No Hot water assumed
2-pipe cooling only No Cold water assumed
4-pipe heat/cool No · Cold water assumed in main coil
· Hot water assumed in aux. coil
The auxiliary electric heat option allows heating when the occupied
space requires heating but the unit still has chilled system water
flowing through it. The electric heat is also sufficient for use as the
sole source of heat for the unit.
The TUC utilizes a heating only output to energize electric heat if
hydronic heat is insufficient to meet the heating requirements. This
output responds to a demand for heat as long as the entering water
temperature is lower than 95° F. However, if the water temperature
reaches 95° F or higher, the electric heat lockout switch (on the
supply water pipe) disengages the electric heat. Electric heat be-
comes available again when the entering water temperature falls to
65° F or lower (± 5° F).